Splitting of Water and the Electron Transport Chain

The photoactivation event at Photosystem II described in the previous section leaves the reaction-centre chlorophyll, now written P680+, in an oxidised state, having lost one of its own electrons. This oxidised P680+ is, in fact, one of the strongest biological oxidising agents known -- strong enough that it is capable of pulling electrons directly out of a water molecule, a reaction water does not undergo easily under ordinary conditions.

This extraction of electrons from water, called the splitting of water or photolysis, is catalysed by a specialised, manganese-containing protein complex, often called the water-splitting complex or oxygen-evolving complex, located on the lumenal (inner) face of the thylakoid membrane, immediately adjacent to Photosystem II. The reaction can be written:

2H₂O → 4H⁺ + O₂ + 4e⁻

This single reaction accomplishes three distinct things simultaneously, each with its own downstream consequence described elsewhere in this chapter. First, it releases electrons, which are fed directly back into Photosystem II, refilling the "electron hole" left by the ejection of the original photoactivated electron and allowing PS II to be re-used for the next photon it absorbs -- without this resupply, PS II would be rapidly exhausted after just one photoactivation event. Second, it releases protons (H+) directly into the thylakoid lumen, where they add to the proton gradient that, as the section on the chemiosmotic hypothesis explains, ultimately drives ATP synthesis. Third, it releases molecular oxygen (O2) as a by-product, which diffuses out of the chloroplast and, eventually, out of the leaf -- this is, quite literally, the very oxygen that Priestley's and Ingenhousz's plants were shown to release, and it is the source of virtually all the free O2 in Earth's atmosphere. …